1. Field of the Invention
The invention relates to power management in communication systems and in particular, adaptive power cutback in DSL communication systems.
2. Description of the Related Art
Telecommunication standards such as ITU G.992.5 define power management in ADSL communication. At the beginning of communication, the transmitter and the receiver go through a process of negotiating various parameters of the communication over the given loop, which is typically referred to as the training period. One of the parameters of communication is the power level at which the transmitter sends its signal. During the training period, the receiver determines the appropriate power level at which the signal should be transmitted. It then communicates this power level to the transmitter and the transmitter may use this power level for later signals that are transmitted to the receiver.
Typically, the receiver balances the transmitted power of the signal with its internal gain to ensure the best data rate and margin on the loop. If the receiver determines that it can increase the internal gain for better signal reception, then the receiver requests the transmitter to cut back power such that the combination of the low power signal and the internal gain of the receiver can still provide the best performance on the loop. The choice of power cutback (PCB) values also has several other functions for example, some DSL standards mandate a reduction in upstream transmitted power based on estimates of the loop length, a receiver may choose PCB values to prevent internal saturation in short loops, a receiver may choose PCB values to reduce the SNR margins in cases where the achievable margins are higher than required and the like.
Generally, PCB values are chosen from lookup tables. These lookup tables are derived in laboratories based on simulations of loop lengths and noise. A typical procedure includes simulating certain loop length and possibly noise conditions in the lab, calculating the best possible value of PCB for those conditions (off-line), storing these values in a lookup table in a modem and iterating these steps for an exhaustive list of loop lengths and noise conditions. When a modem is deployed in the field, it can estimate the loop length and noise condition and determine the corresponding choice of PCB from the pre-stored lookup table. However, several problems exist with this approach. For example, it is impractical to determine a completely exhaustive list of loop lengths and noise conditions due to memory constraints in each modem, the value of noise conditions are pre-determined and so they do not account for dynamic changes in conditions such as environmental changes in the chip and the like. Further, the process of calculating the look-up tables needs to be repeated whenever there are changes made to the modem such as silicon revisions, component changes, software changes affecting noise performance etc. Due to these factors, existing modems use approximations in many cases. These approximations may lead to sub-optimal performance.
Therefore, it is desirable to implement an adaptive power cutback algorithm in modems that can estimate the line conditions and dynamically calculate the optimal PCB value for the given conditions.